Stance and sensory feedback influence on postural dynamics

This study examined the effects of ice-induced plantar desensitization and the withdrawal of visual feedback on the magnitude and time-dependent structure of postural sway variability. The magnitude of variability was quantified as the area of an ellipse enclosing 95% of the center of pressure (COP) time-series during normal and tandem stances. The same time-series were also analyzed using Approximate Entropy (ApEn) and Cross-Approximate Entropy (CrossApEn) as indices of irregularity and asynchrony between the mediolateral and anteroposterior COP motions. Variability increased during tandem stance and this increase was compounded by both visual feedback withdrawal and cutaneous desensitization. Both ApEn (mediolateral and anteroposterior COP motion) and CrossApEn increased with the withdrawal of visual feedback during the tandem stance, but decreased significantly during normal stance. The results of the study demonstrate that plantar desensitization only affected the magnitude of sway variability but did not alter its time-dependent structure. Contrasting effects on the structure of postural sway variability with visual feedback withdrawal were observed during the different stances, highlighting the role of task demands in postural dynamics.     

Exploring coordination dynamics of the postural system with real-time visual feedback

Differential performance over a wide range of possible postural coordination modes was investigated using 16 ankle-hip relative phase patterns from 0 degrees to 337.5 degrees. Participants were instructed to produce each coordination mode with and without real time visual feedback. Feedback consisted of a Lissajous figure indicating the discrepancy between actual and requested ankle-hip relative phase. The results showed: (1) the presence of a unique attractor around the anti-phase pattern (relative phase approximately 180 degrees); (2) performance was similar with and without visual feedback; (3) the absence of an attractor for the in-phase pattern (relative phase approximately 20 degrees). The third result is not consistent with previous research in which both in-phase and anti-phase patterns emerged when they were not imposed [B.G. Bardy, L. Martin, T.A. Stoffregen, R.J. Bootsma, Postural coordination modes considered as emergent phenomena, J. Exp. Psychol. Hum. Percept. Perform. 25 (1999) 1284-1301; B.G. Bardy, O. Oullier, R.J. Bootsma, T.A. Stoffregen, Dynamics of human postural transitions, J. Exp. Psychol. Hum. Percept. Perform. 28 (1999) 499-514]. This finding indicates the strong dependency to task variation and instructions of postural pattern formation.     

Movement sway: changes in postural sway during voluntary shifts of the center of pressure

We introduce a method for quantification of movement sway—spontaneous migrations of the center of pressure (COP) during its voluntary shifts. Subjects stood on a force platform or on a board with a narrow support surface ("unstable board") and performed voluntary cyclic shifts of the COP at different frequencies. Movement sway was typically higher than postural sway; sway in the mediolateral direction was particularly increased. Movement sway showed a drop with the frequency of voluntary COP shifts. During standing on the unstable board, postural sway increased while movement sway decreased. The effects of task parameters were stronger on the sway component in the direction of the voluntary COP shift than in the orthogonal direction. We interpret changes in movement sway with task parameters as partly resulting from modulation of the search function of sway during voluntary COP shifts. Electronic Publication     

Task-specific adaptations of postural sway in sitting infants

When engaging in manual or visual tasks while sitting, infants modify their postural sway based on concurrent task demands. It remains unclear whether these modulations are sensitive to differences in concurrent task demands (holding a toy vs. looking at a toy being held by someone else), and whether the properties of the support surface impact these adaptations. We investigated infants’ ability to modify postural sway when holding a toy or visually attending to a toy someone else was holding while sitting on different support surfaces. Twenty-six independently sitting infants sat on solid and compliant surfaces placed on a force plate while looking at or holding a toy. Measures of postural sway were calculated from the center of pressure data. Visually attending to a toy was associated with less sway and lower sway velocity than when holding a toy. Surprisingly, surface compliance did not affect sway and there were no interaction effects. Whereas sway modulations may facilitate infants’ performance on both manual and visual concurrent tasks, the visual task placed more constraints on the postural system leading to greater adaptations in postural sway. These findings provide insights into how infants are allocating attention and coordinating perceptual-motor information in developing sitting skills.     

The spectral characteristics of postural sway behaviour

Summary To determine whether recognisable profiles of sway behaviour are characteristically found among normal subjects the energy content of a contiguous series of bandwidths were computed from power density spectra of the sway behaviour of 29 young men and 29 young women. No significant differences were observed over time either in the energy content or in the profile shape of the power spectral density (PSD) records when these were subjected to a multivariate profile analysis of variance. It was observed that the majority of energy in the power spectrum, viz. 90% (±4%) in the antero-posterior and 95% (±3%) in the lateral direction, was below 2.0 Hz. Three dominant frequencies of sway were identifiable in 93% of subjects. The modal frequencies in the antero-posterior direction were 0.30–0.45 Hz (primary), 0.60–0.75 Hz (secondary) and 1.05–1.20 Hz (tertiary), and in the lateral direction 0.30–0.45 Hz (primary), 0.45–0.60 Hz (secondary) and 0.75–0.90 Hz (tertiary). These three dominant frequencies were distributed over the full frequency range. An explanation of their origins was not sought but the suggestion is made that they could be driven by common dynamic physiological events. The patterns of energy distribution were simplified and quantified by the calculation of a difference-ratio index. The broad dispersion of the values of this index precludes the suggestion that there could be any clustering of profiles. It is concluded that the frequently postulated clustering of the sway patterns of young adults into recognisable and distinct profiles is without firm foundation. Instead it is suggested that sway behaviour is probably normally and randomly distributed about common respiratory, cardiac and perhaps also other dynamic physiological events. This work has been carried out with the support of the Science Research Council     

Spontaneous postural sway predicts the strength of smooth vection

This study asked whether individual differences in the influence of vision on postural stability could be used to predict the strength of subsequently induced visual illusions of self-motion (vection). In the experiment, we first measured spontaneous postural sway while subjects stood erect for 60 s with their eyes both open and both closed. We then showed our subjects two types of self-motion display: radially expanding optic flow (simulating constant velocity forwards self-motion) and vertically oscillating radially expanding optic flow (simulating constant velocity forwards self-motion combined with vertical head oscillation). As expected, subjects swayed more with their eyes closed (compared to open) and experienced more compelling illusions of self-motion with vertically oscillating (as opposed to smooth) radial flow. The extent to which participants relied on vision for postural stability—measured as the ratio of sway with eyes closed compared to that with eyes open—was found to predict vection strength. However, this was only the case for displays representing smooth self-motion. It seems that for oscillating displays, other factors, such as visual–vestibular interactions, may be more important.     

Individuals with post-stroke hemiparesis are able to use additional sensory information to reduce postural sway

The present study aimed to investigate whether stroke survivals are able to use the additional somatosensory information provided by the light touch to reduce their postural sway during the upright stance. Eight individuals, naturally right-handed pre-stroke, and eight healthy age-matched adults stood as quiet as possible on a force plate during 35 s. Participants performed two trials for each visual condition (eyes open and closed) and somatosensory condition (with and without the right or left index fingertip touching an instrumented rigid and fixed bar). When participants touched the bar, they were asked to apply less than 1 N of vertical force. The postural sway was assessed by the center of pressure (COP) displacement area, mean amplitude and velocity. In addition, the mean and standard deviation of the force vertically applied on the bar during the trials with touch were assessed. The averaged values of COP area, amplitude and velocity were greater for stroke individuals compared to healthy adults during all visual and somatosensory conditions. For both groups, the values of all variables increased when participants stood with eyes closed and reduced when they touched the bar regardless of the side of the touch. Overall, the results suggested that, as healthy individuals, persons with post-stroke hemiparesis are able to use the additional somatosensory information provided by the light touch to reduce the postural sway.     

Anxiety affects the postural sway of the antero-posterior axis in college students

To examine whether the postural balance is influenced by the degree of anxiety, body sway during orthostatic standing while gazing at a visual target was examined in college students. Students, physically and mentally healthy, were divided into two groups according to the degree of state anxiety; high anxiety group (HA) and low anxiety group (LA). A fast Fourier transform analysis of the postural sway in antero-posterior axis showed that frequency components of 0.02-0.21 Hz, reflecting vestibular inputs, were 16% greater and those of 2.02-10.0 Hz, reflecting somatosensory inputs, were 24% smaller in HA. These differences between HA and LA were abolished when the eyes were closed. It is concluded that the interactions of visual inputs with vestibular and somatosensory inputs are influenced by anxiety.     

Changes in postural sway as a function of prolonged walking

For optimal balance, the postural system needs to quickly detect and respond to perturbations. The aim of this study was to investigate the immediate and long-term impact of walking at different speeds on standing balance and postural stability. Center of pressure (COP) motion was measured from 14 young individuals at discrete time intervals after they walked on a treadmill at three speeds (preferred walking speed (PWS), 120 %-PWS, 140 %-PWS). Results revealed that walking at a faster speed had the greatest impact on postural stability. This was reflected by increases in the amount (path length, range, 95 % ellipse), variability (standard deviation, SD), and structure (approximate entropy, ApEn) of COP motion and were most evident when compared to pre-walking assessments. In subsequent trials following pre-walking assessments there was a leveling-off for specific COP variables (range, variability, and ApEn) and a decline in path length. This plateau effect was observed even though measures of physical exertion (HR, RPE) continued to increase over the entire walking trial. Together, these results indicate that, despite the constant task demands induced by fast walking, the postural system was able to rapidly compensate and adjust appropriately.     

Contributions of delayed visual feedback and cognitive task load to postural dynamics

In this experiment, we examined the extent to which postural control is influenced by visual and cognitive task performance. Fourteen healthy young participants performed a balance task in eyes-open (EO) and delayed visual feedback (DVF) conditions. DVF was presented at delays ranging from 0 to 1200 ms in 300 ms increments. Cognitive load was implemented by a simple, serial arithmetic task. High and low-pass filtering (f_c = 0.3 Hz) distinguished LOW and HIGH frequency components, which were used to compute the variability of Anteroposterior (AP) Center of Pressure (COP) trajectories on fast (>0.3 Hz) and slow (<0.3 Hz) times cales. Imposed visual delay increased sway variability at both LOW and HIGH components. Cognitive task performance, however, influenced only the variability of fast (HIGH) sway components. Our results support distinct timescale mechanisms for postural control, but also demonstrate that vision predominantly influences low frequency components of postural sway. Moment-to-moment COP fluctuations are dependent on cognitive performance during delayed visual feedback postural control.     

Reversals of anticipatory postural adjustments during voluntary sway in humans

We describe reversals of anticipatory postural adjustments (APAs) with the phase of a voluntary cyclic whole-body sway movement. Subjects ( n = 9) held a standard load in extended arms and released it by a bilateral shoulder abduction motion in a self-paced manner at different phases of the sway. The load release task was also performed during quiet stance in three positions: in the middle of the sway range and close to its extreme forward and backward positions. Larger APAs were seen during the sway task as compared to quiet stance. Although the direction of postural perturbation associated with the load release was always the same, the direction of the APAs in the leg muscles reversed when the subjects were close to the extreme forward position as compared to the APAs in other phases and during quiet stance. The trunk muscles showed smaller APA modulation at the extreme positions but larger modulation when passing through the middle position, depending on the direction of sway, forward or backward. The phenomenon of APA reversals emphasizes the important role of safety in the generation of postural adjustments associated with voluntary movements. Based on these findings, APAs could be defined as changes in the activity of postural muscles associated with a predictable perturbation that act to provide maximal safety of the postural task component.     

Automatic control of postural sway by visual motion parallax

The purpose of this study was to establish whether visual motion parallax participates in the control of postural sway. Body sway was measured in ten normal subjects by photoelectric recordings of head movements and by force-plate posturography. Subjects viewed a visual display (“background”), which briefly moved (2 s) along the y (horizontal) axis, under three different conditions: (1) direct fixation of the background, (2) fixation of a stationary window frame in the foreground, and (3) fixation of the background in the presence of the window in the foreground (“through the window”). In response to background fixation, subjects swayed in the same direction as stimulus motion, but during foreground (window) fixation they swayed in the opposite direction. The earlier forces observed on the force platform occurred at circa 250 ms in both conditions. The results show that motion parallax generates postural responses. The direction of these parallax-evoked postural responses — opposite to other visually evoked postural responses reported so far — is appropriate for stabilizating posture in natural circumstances. The findings show that motion parallax is an important source of self-motion information and that this information participates in the process of automatic postural control. In the “fixating through the window” condition, which does not mimic visual conditions induced by body sway, no consistent postural responses were elicited. This implies that postural reactions elicited by visual motion are not rigid responses to optokinetic stimulation but responses to visual stimuli signalling self-motion.     

Intertester reliability of assessing postural sway using the chattecx balance system

In this study, we examined intertester reliability of dynamic and static balance using the Chattecx Dynamic Balance System. Ten female and two male subjects were randomly assigned to two testers and completed ten, 10-second preprogrammed double- and single-leg test conditions. Balance was measured as postural sway in centimeters by the Chattecx Dynamic Balance System. Static balance consisted of the platform remaining stable, whereas dynamic balance consisted of the platform tilting in an anterior and posterior direction during testing. The protocols and average postural sway values were:1. Double-Leg Static Eyes Open ([unk] = .28 cm)2. Double-Leg Static Eyes Closed ([unk] = .37 cm)3. Double-Leg Dynamic Eyes Open ([unk] = .86 cm)4. Double-Leg Dynamic Eyes Closed ([unk] = 1.72 cm)5. Single-Leg Static Dominant Eyes Open ([unk] = .99 cm)6. Single-Leg Static Dominant Eyes Closed ([unk] = 1.70 cm)7. Single-Leg Static Nondominant Eyes Open ([unk] = .65 cm)8. Single-Leg Static Nondominant Eyes Closed ([unk] = 1.48 cm)9. Single-Leg Dynamic Dominant Eyes Open ([unk] = .99cm)10. Single-Leg Dynamic Nondominant Eyes Open ([unk] = .91 cm).The mean values of postural sway ranged from .28 cm to 1.72 cm for double-leg stance measures and from .65 cm to 1.70 cm for the single-leg stance measures. The intraclass correlations (ICCs)(2,1) ranged from poor to excellent (ICCs = .06 to .90) and the standard error of measurement (SEM) ranged from .06 to .34 for the 10 trials. We feel that variability exists between subjects and/or testers between trials. The wide range of reliability values suggests that further research should determine both intratester and intertester reliability using a variety of protocols for assessment of static and dynamic balance.     

Recurrence analysis of human postural sway during the sensory organization test

The present study examined how the availability of and alterations in sensory information during the sensory organization test (SOT) influenced the amount, variability, and temporal structure of spontaneous postural sway in young, healthy adults. Findings indicated that postural sway tended to increase in amount and variability as the SOT condition became increasingly difficult (i.e. as the SOT condition moved from eyes open to eyes closed, to sway-referenced visual surround or support surface, and to sway-reference surface and visual surround). In addition, recurrence quantification analysis revealed that the temporal structure of postural sway tended to become increasingly regular as the SOT condition increased in difficulty. The functional utility of the observed changes in the temporal structure of postural sway across sensory conditions was discussed.     

Control of the triceps surae during the postural sway of quiet standing

AIM: The present study investigated how the triceps surae are controlled at the spinal level during the naturally occurring postural sway of quiet standing. METHODS: Subjects stood on a force platform as electrical stimuli were applied to the posterior tibial nerve when the center of pressure (COP) was either 1.6 standard deviations anterior (COP(ant)) or posterior (COP(post)) to the mean baseline COP signal. Peak-to-peak amplitudes of the H-reflex and M-wave from the soleus (SOL) and medial gastrocnemius (MG) muscles were recorded to assess the efficacy of the Ia pathway. RESULTS: A significant increase in the H(max) : M(max) ratio for both the SOL (12 +/- 6%) and MG (23 +/- 6%) was observed during the COP(ant) as compared to the COP(post) condition. The source of the modulation between COP conditions cannot be determined from this study. However, the observed changes in the synaptic efficacy of the Ia pathway are unlikely to be simply a result of an altered level of background electromyographic activity in the triceps surae. This was indicated by the lack of differences observed in the H(max) : M(max) ratio when subjects stood without postural sway (via the use of a tilt table) at two levels of background activity. CONCLUSIONS: It is suggested that the phase-dependent modulation of the triceps surae H-reflexes during the postural sway of quiet standing functions to maintain upright stance and may explain the results from previous studies, which, until now, had not taken the influence of postural sway on the H-reflex into consideration.     

Reduced postural sway during quiet standing by light touch is due to finger tactile feedback but not mechanical support

It is well known that a light and voluntary touch with a fingertip on a fixed surface improves postural stability during quiet standing. To determine whether the effect of the light touch is due to the tactile sensory input, as opposed to mechanical support, we investigated the light touch effect on postural stability during quiet standing with and without somatosensory input from the fingertip. Seven young subjects maintained quiet standing on a force platform with (LT) and without (NT) lightly touching a fixed surface, and with (TIS) and without (CON) the application of tourniquet ischemia, which removed the tactile sensation from the fingertip. The mean velocity of centre of pressure (CoP) was calculated to assess the postural sway in each condition. The mean velocity of CoP was significantly smaller in the LT condition compared to the NT condition only under the CON condition, whereas the light touch effect was not significant under the TIS condition. We found that the reduction of the horizontal ground reaction force due to the light touch was about 20%, which was approximately equivalent to the reduction of mean velocity of CoP in the LT condition compared to the NT condition. Since the fingertip contact force was relatively large compared to the horizontal ground reaction force, one could say that the light touch effect might be due to the mechanical support provided by the contact itself. However, we demonstrated experimentally that light touch effects were diminished due to loss of finger tactile feedback induced by the tourniquet ischemia, but not due to the mechanical support provided by the light touch. One possible reason is the lack of feedback information in controlling posture, and the other is the altered control of the arm induced by the loss of tactile feedback.     

Analysis of postural sway using entropy measures of signal complexity

A stochastic complexity analysis is applied to centre-of-pressure (COP) time series, by using different complexity features, namely the spectral entropy, the approximate entropy, and the singular value decomposition spectrum entropy. A principal component analysis allows an estimate of the overall signal complexity in terms of the ensemble complexity score; the difference in values between open-eyes (OE) and closed-eyes (CE) trials is used for clustering purposes. In experiments on healthy young adults, the complexity of the mediolateral component is shown not to depend on the manipulation of vision. Conversely, the increase of the anteroposterior complexity in OE conditions can be statistically significant, leading to a functional division of the subjects into two groups: the Romberg ratios (RRs), namely the ratios of the CE measure to the OE measure, are: RR=1.19±0.15 (group 1 subjects), and RR=1.05±0.14 (group 2 subjects). Multivariate statistical techniques are applied to the complexity features and the parameters of a postural sway model recently proposed; the results suggest that the complexity change is the sign of information-generating behaviours of postural fluctuations, in the presence of a control strategy which aims at loosening long-range correlation and decreasing stochastic activity when visual feedback is allowed.     

Dynamic patterns of postural sway in ballet dancers and track athletes

We compared the variability and spatiotemporal profile of postural sway of trained ballet dancers to college varsity track athletes under variations in the availability of vision and rigidity of the support surface. We found no differences between the groups according to the variability measures, but variability increased for both groups with eyes closed and on a foam surface. Recurrence quantification analysis revealed that the postural sway of dancers was less regular (lower recurrence), less stable (lower maxline), less complex (lower entropy), and more stationary (lower absolute trend) than that of track athletes. Dancers, possibly as a result of focused balance training, exhibited different dynamic patterns of postural sway. This research was part of a senior honors thesis by the second author.     

Effects of attentional focus on postural sway in children and adults

The present study examined, in children aged 4–11 and in adults, the postural control modifications when attention was oriented voluntary on postural sway. Since (1) there are less attentional resources in children than in adults, (2) the selective attention processing improves with age, i.e., children use a different strategy to focus their attention than adults, and (3) adults’ postural stability decreases when attention is focused on postural sway, we hypothesized that postural stability was less affected in children than in adults when attention was focused on postural sway. Fourty four children aged 4- to 11-year-old and 11 adults participated in the experiments. The postural control task was executed in a Romberg position. Two experimental conditions were presented to the subjects, (1) to look at a video on a TV screen without instruction about the posture, and (2) to fixate a cross placed at the center of the TV screen with the instruction to remain as stable as possible. Postural performance was measured by means of a force platform. Results from this study (1) confirmed a non-monotonic improvement of postural stability during the ontogenetic period without reaching the adults’ level at the age of 11, (2) suggested that children, aged 4–11, are able to focus their attention on the control of posture, and (3) showed that the automatic control of posture increases postural stability since the age of 4.